Battles are fought under different weather conditions, which may even change during the battle itself. The current weather is displayed as an icon at the top of the screen. If you hover the mouse above that icon, a popup text will explain what effects the weather has on the various unit types.
If you see an exclamation mark next to the weather display, that means there are also special circumstances influencing the battle. These can be beneficial – such as increasing mana regeneration – or impeding. Like the weather, special circumstances affect both sides all the same.